*2.2.1 Nitrogen*

Yeasts use the nutrients of the must for their growth during fermentation process. Here, nitrogen is essential to develop reactions that will derive in the formation of secondary metabolites which are very important on the quality of the wine, such as glycerol, organic acids (lactic, acetic, succinic), esters, sulfur compounds or amino acids released during this phenomenon. Usually, during the alcoholic fermentation, nitrogen is added during the exponential phase of fermentation that corresponds to the growth of the yeast (first days of fermentation). Nitrogen can be assimilated by yeasts during winemaking in two different forms, as ammonium or as amino acids. In this phase it has an effect on cell growth and on the rate of fermentation [12]. In musts with few nutrients, the amount of assimilable nitrogen drops early and induces the production of hydrogen sulfide (H2S) due to the absence of compounds that capture sulfur.

Wine yeasts can form H2S from inorganic sulfur compounds (sulfate or sulfite), or organic sulfur compounds (cysteine, methionine or glutathione). The production of H2S can occur from the Sulfate Reduction Sequence (SRS) route, where sulfate is used for the biosynthesis of cysteine and methionine. Sulfate is accumulated from the medium, and then reduced to sulfite following sulfite is reduced by sulfite reductase to sulfide. Therefore, when dealing with nitrogen-deficient musts, yeast will tend to synthetize it from nitrogenous precursors *o*-acetilserina and *o*-acetilhomoserina, with which the sulfite produced will be excreted as hydrogen sulfide (H2S) [13]. Therefore, in wines with a limited content of nitrogen the supplementation with sulfur amino acids the production of hydrogen sulfide can increase considerably. H2S is a highly reactive compound, and it can combine with different components present in wine forming other VSCs [14]. Mercaptans, sulfides and disulfides can be also found in wines.

In many cases, H2S production can be controlled by adding nitrogenous salts such as diammonium phosphate (DAP). Some studies suggest that a concentration of 200–250 mg/L of assimilable nitrogen is necessary to minimize the risk of H2S production. However, not all commercial strains show the same behavior to the improvement of the must by the addition of diammonium phosphate, and usually indicates a deficiency in the juice of one or more vitamins, pantothenic acid, pyridoxine or biotin, which is involved in the metabolism of H2S. The persistence of H2S production problems, even with nutrient supplementation, requires the selection of yeast with low H2S production in such musts.

It has been reported that some strains appear to produce H2S inherently without being affected by environmental conditions, possibly indicating a metabolic defect [15, 16]. Therefore, the H2S production capacity of a specific strain has a genetic influence, since the H2S production of different strains varies under the same conditions [13, 16, 17]. The excessive production of hydrogen sulfide that takes place during the fermentation process is a fairly common problem in winemaking [13, 17]. As mentioned, the persistence of H2S production problems, even with nutrient supplementation, requires the selection of yeast strains with low H2S production. New yeast strains have been developed to produce undetectable amounts of H2S [18]. In summary, yeasts and nutrients, such as the nitrogen content have a manifest influence on the different metabolites produced during fermentation, many of them with a very clear impact on the wine aroma and therefore in the LST default.

#### *2.2.2 Riboflavin*

Riboflavin acts as a photosensitizer in many foods and beverages. The RF level in grapes is usually less than a few tens of micrograms per liter of must [19], but can increase during winemaking mainly due to the metabolic activity of *Saccharomyces cerevisiae* [20]. Values close to 150 μg/L or even higher can eventually occur in wine depending on the yeast strain used for the alcoholic fermentation [21, 22]. Riboflavin-producing yeast strains have occasionally been found to be methionineproducing as well, which may increase the risk of spoilage [21]. The amount of methionine oxidized in wine exposed to light is related to several physical and chemical factors, including the concentration of riboflavin, oxygen, and other amino acids. Photosensitized RF can oxidize methionine as well as other amino acids. The reduced riboflavin can then be oxidized back to riboflavin by oxygen [23]. It is also known that the presence of riboflavin in wine is mainly due to the metabolism of the yeast *Saccharomyces cerevisiae*. Some *Saccharomyces* strains can prevent a high amount of riboflavin in wine [21]. Yeast is known to contain a gene, RIB5, which encodes the formation of the enzyme riboflavin synthase, which is involved in the last step of RF synthesis by yeast [20]. The use of yeast strains that have a lower capacity to produce riboflavin may be a potential means of minimizing its concentration in wine.

Some studies carried out at our facilities in the Wine Technology Center (VITEC) reported the importance in the use of different yeasts and nutrients to carry out the fermentation to diminish the RF content in wine (**Figure 2**). Different types of commercial *Saccharomyces cerevisiae* strains were assessed with different types of nutrition during fermentation. In this case, one of the *S. cerevisiae* strain used (yeast strain 2) produced higher RF content in three of the four studied nutrition conditions. In addition, nutrition 1 and especially nutrition 3 increased noticeably the production of RF. This could be explained by differences on the metabolism of each strain and the characteristics of the nutrients. These two conditions of nutrition were based on yeast cell walls, richer in vitamins while nutrition 4 was based in inorganic addition by DAP.

**Figure 2.**

*Production of riboflavin with two different strains of* Saccharomyces cerevisiae *through four different types of nutrition.*

#### *The Light Struck Taste of Wines DOI: http://dx.doi.org/10.5772/intechopen.99279*

extract-based nutrients often contain vitamins, including RF, which can therefore increase its amount content in wine. The use of low RF products has been proposed to prevent the formation of volatile sulfur compounds. Yeast lysate can also be used as an additive to prevent the anti-fermentative activity of medium chain fatty acids [19]. The lipid fraction naturally found in yeast lysate may have affected the ability of fermenting yeast to produce purines, the precursors of riboflavin in yeast metabolism [24, 25].
